Data Communications and
Networks
Chapter 7 – IP Addressing
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Agenda
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•
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•
•
•
•
•
•
Objective;
What is an IP Address?
IP Address;
Network Prefix and Host Number;
Subnetting;
Subnetmask;
IP Address classes;
CIDR;
Routing;
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Objective
By the end of this lesson, the student will
understand how the IP addresses work and how to
calculate a subnet address.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Pre-assessment
What do you know about?
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•
•
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IP Address;
Subnetting;
Subnetmask;
CIDR.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
What is an IP Address?
• An IP address is a unique global address for a
network interface
• Exceptions:
– Dynamically assigned IP addresses ( DHCP)
– IP addresses in private networks ( NAT)
• An IP address:
- is a 32 bit long identifier
- encodes a network number (network
prefix) and a host number
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
IP Addresses
This allocation has come
to be called classful
addressing. It is no
longer
used,
but
references to it in the
literature
are
still
common. We will discuss
the replacement of
classful
addressing
shortly
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
IP Addresses
32 bits
version
(4 bits)
header
length
Type of Service/TOS
(8 bits)
flags
(3 bits)
Identification (16 bits)
TTL Time-to-Live
(8 bits)
Total Length (in bytes)
(16 bits)
Protocol
(8 bits)
Fragment Offset (13 bits)
Header Checksum (16 bits)
Source IP address (32 bits)
Destination IP address (32 bits)
Ethernet Header
IP Header
TCP Header
Application data
Ethernet Trailer
Ethernet frame
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
IP Addresses
32 bits
0x4
0x5
0x00
9d08
12810
4410
0102
00000000000002
0x06
8bff
128.143.137.144
128.143.71.21
Ethernet Header
IP Header
TCP Header
Application data
Ethernet Trailer
Ethernet frame
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Network prefix and host number
• The network prefix identifies a network and the host
number identifies a specific host (actually, interface on the
network).
• How do we know how long the network prefix is?
– Before 1993: The network prefix is implicitly defined (see classbased addressing)
or
– After 1993: The network prefix is indicated by a netmask.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Dotted Decimal Notation
• IP addresses are written in a so-called dotted
decimal notation
• Each byte is identified by a decimal number in
the range [0..255]:
10000000
10001111
10001001
10010000
1st Byte
2nd Byte
3rd Byte
4th Byte
= 143
= 137
= 144
= 128
128.143.137.144
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Subnetting
• Problem: Organizations
have multiple networks
which are independently
managed
– Solution 1: Allocate a separate
network address for each network
• Difficult to manage
• From the outside of the
organization, each network
must be addressable.
another
level of hierarchy to the
IP addressing structure
CollegeNetwork
IT
School
Medical
School
Library
– Solution 2: Add
ICT-BVF8.1- Data Communications and Network
Subnetting
Trainer: Dr. Abbes Sebihi
Address assignment with subnetting
• Each part of the organization is allocated a range of IP addresses (subnets or
subnetworks)
• Addresses in each subnet can be administered locally
128.143.0.0/16
CollegeNetwork
128.143.71.0/24
128.143.136.0/24
IT
School
Medical
School
128.143.56.0/24
Library
128.143.121.0/24
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Basic Idea of Subnetting
• Split the host number portion of an IP address into a
subnet number and a (smaller) host
number.
• Result is a 3-layer hierarchy
network prefix
network prefix
• Then:
host number
subnet number
host number
extended network prefix
• Subnets can be freely assigned within the organization
• Internally, subnets are treated as separate networks
• Subnet structure is not visible outside the organization
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Subnetmask
• Routers and hosts use an extended network prefix
(subnetmask) to identify the start of the host
numbers 128.143
137.144
host number
network prefix
128.143
137
144
network prefix
subnet
number
host number
extended network prefix
1111111111111111 1111111100000000
subnetmask
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Advantages of Subnetting
• With subnetting, IP addresses use a 3-layer hierarchy:
» Network
» Subnet
» Host
• Reduces router complexity. Since external routers do
not know about subnetting, the complexity of routing
tables at external routers is reduced.
• Note: Length of the subnet mask need not be identical
at all subnetworks.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Example: Subnetmask
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•
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128.143.0.0/16 is the IP address of the network
128.143.137.0/24 is the IP address of the subnet
128.143.137.144 is the IP address of the host
255.255.255.0 (or ffffff00) is the subnetmask of the host
When subnetting is used, one generally speaks of a “subnetmask” (instead
of a netmask) and a “subnet” (instead of a network)
• Use of subnetting or length of the subnetmask if decided by the network
administrator
• Consistency of subnetmasks is responsibility of administrator
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
The old way: Internet Address Classes
bit # 0
Class A
1
7 8
31
0
Network Prefix
Host Number
8 bits
24 bits
bit # 0 1 2
Class B
10
15 16
network id
host
Network Prefix
Host Number
16 bits
16 bits
bit # 0 1 2 3
Class C
31
110
ICT-BVF8.1- Data Communications and Network
23 24
network id
31
host
Network Prefix
Host Number
24 bits
8 bits
Trainer: Dr. Abbes Sebihi
The old way: Internet Address Classes
bit # 0 1 2 3 4
Class D
1110
31
multicast group id
bit # 0 1 2 3 4 5
Class E
11110
31
(reserved for future use)
• We will learn about multicast addresses later in
this course.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
CIDR - Classless Interdomain Routing
• IP backbone routers have one routing table entry
for each network address:
– With subnetting, a backbone router only needs to know one entry for
each Class A, B, or C networks
– This is acceptable for Class A and Class B networks
• 27 = 128 Class A networks
• 214 = 16,384 Class B networks
– But this is not acceptable for Class C networks
• 221 = 2,097,152 Class C networks
• In 1993, the size of the routing tables started to
outgrow the capacity of routers
• Consequence: The Class-based assignment of IP
addresses had to be abandoned
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
CIDR - Classless Interdomain Routing
• Goals:
– New interpretation of the IP address space
– Restructure IP address assignments to increase efficiency
– Permits route aggregation to minimize route table entries
• CIDR (Classless Interdomain routing)
– abandons the notion of classes
– Key Concept: The length of the network prefix in the IP
addresses is kept arbitrary
– Consequence: Size of the network prefix must be provided
with an IP address
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
CIDR Notation
• CIDR notation of an IP address:
192.0.2.0/18
• "18" is the prefix length. It states that the first 18 bits are the
network prefix of the address (and 14 bits are available for specific
host addresses)
• CIDR notation can replace the use of subnetmasks (but is more general)
– IP address 128.143.137.144 and subnetmask 255.255.255.0 becomes
128.143.137.144/24
• CIDR notation allows to drop traling zeros of network addresses:
192.0.2.0/18 can be written as 192.0.2/18
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
CIDR Notation
CIDR notation
Network mask
Available
Available
Total
subnets
hosts per subnet
usable
hosts
/24
255.255.255.0
1
254
254
/25
255.255.255.128
2
126
252
/26
255.255.255.192
4
62
248
/27
255.255.255.224
8
30
240
/28
255.255.255.240
16
14
224
/29
255.255.255.248
32
6
192
/30
255.255.255.252
64
2
128
/31
255.255.255.254
128
2*
256
Data Communications and Network
CIDR and Routing
• Aggregation of routing table entries:
– 128.143.0.0/16 and 128.144.0.0/16 are represented as 128.142.0.0/15
• Longest prefix match: Routing table lookup finds the routing entry that
matches the longest prefix
What is the outgoing interface for
128.143.137.0/24 ?
Prefix
Interface
128.0.0.0/4
interface #5
128.128.0.0/9
interface #2
128.143.128.0/17 interface #1
Route aggregation can be exploited
when IP address blocks are assigned
in an hierarchical fashion
ICT-BVF8.1- Data Communications and Network
Routing table
Trainer: Dr. Abbes Sebihi
Summary
Every host and router on the Internet has an IP address, which encodes its
network number and host number. The combination is unique: in principle, no
two machines on the Internet have the same IP address.
The IP addressing calculation is very important for a network administrator,
because it gives you the knowledge necessary for designing or configuring a
network.
ICT-BVF8.1- Data Communications and Network
Trainer: Dr. Abbes Sebihi
Questions
Data Communications and Network